Image sensor capable of eliminating rolling flicker and adjusting frame rate

ABSTRACT

There is provided an image sensor for exposing a plurality of pixel rows within a frame period using a rolling shutter. The image sensor includes a processor for calculating bright-dark distribution patterns of image frames. The processor further adjusts the frame period to be substantially identical to a predetermined period by changing a total number of exposed line times within the frame period when a difference between the bright-dark distribution patterns of two image frames is larger than a predetermined threshold.

BACKGROUND 1. Field of the Disclosure

This disclosure generally relates to an image sensor and, moreparticularly, to an image sensor capable of compensating the clockshifting caused by environmental change and eliminating the rollingflicker in image frames.

2. Description of the Related Art

The optical motion sensor identifies the variation of object positionswith time by detecting reflected light at different times from anobject. In the motion sensor using the rolling shutter, as every pixelrow starts to expose at different times, the flicker can appear in imageframes acquired by an image sensor when ambient light has an intensityvariation of a specific frequency (e.g., under a fluorescent lamp).Especially when a frame rate of the image sensor does not match theintensity variation frequency, so called moving flickers or rollingflickers appear in the image frames to degrade the accuracy of motiondetection.

This mismatch can be caused by environmental change, e.g., the clockfrequency of the motion sensor being shifted due to the temperaturevariation. This clock frequency change leads to an actual frame rate ofthe image sensor during operation not identical to the expected framerate.

Accordingly, the present disclosure provides an image sensor that caneliminate rolling flickers in the image frames even using an incorrectclock frequency.

SUMMARY

The present disclosure provides an image sensor that fine tunes theframe rate to match an oscillation frequency of ambient light toeliminate the rolling flicker even using an incorrect clock frequency.In this way, the detection accuracy is improvable even withoutcorrecting the clock frequency or using an accurate crystal oscillator.

The present disclosure further provides an image sensor that causes theframe rate to match a predetermined frequency by adjusting a blank timeinterval between two adjacent image frames. In this way, even though theclock frequency drifts with the environmental change, the image framesare outputted with a correct frame rate.

The present disclosure provides an image sensor acquiring image frameswith a frame period and including a pixel array and a processor. Thepixel array is configured to output a first image frame and a secondimage frame, wherein the pixel array is exposed using a rolling shutter.The processor is configured to calculate a first bright-darkdistribution pattern of the first image frame, calculate a secondbright-dark distribution pattern of the second image frame, and comparethe first bright-dark distribution pattern and the second bright-darkdistribution pattern to confirm existence of rolling flickers.

The present disclosure further provides an image sensor acquiring imageframes with a frame period and including a pixel array and a processor.The pixel array is configured to output a first image frame and a secondimage frame, wherein the pixel array is exposed using a rolling shutter,and the frame period comprises line times of multiple valid pixel rowsand line times of multiple dummy pixel rows. The processor is configuredto calculate a first bright-dark distribution pattern of the first imageframe, calculate a second bright-dark distribution pattern of the secondimage frame, and adjust a blank time according to a difference betweenthe first bright-dark distribution pattern and the second bright-darkdistribution pattern.

The present disclosure further provides an image sensor acquiring imageframes with a frame period and including a pixel array, a clockgenerator and a processor. The pixel array is configured to outputsuccessive image frames using a rolling shutter, wherein the frameperiod of the image frames comprises line times of multiple valid pixelrows and line times of multiple dummy pixel rows. The clock generator isconfigured to use a clock frequency to count each of the line times ofmultiple valid pixel rows and the line times of multiple dummy pixelrows. The processor is configured to adjust a blank time to compensate adeviation of the frame period caused by a clock frequency shift when theclock frequency of the clock generator is changed by a temperaturevariation.

In the present disclosure, the dummy pixel rows are referred to pixelrows that do not physically exist and are controlled by dummy rowaddress. A next frame period is entered after the line time of the dummypixel rows is ended. The line time of the dummy pixel rows is used tocreate a waiting time interval or a blank time interval after physicalpixel rows are all exposed so as to extend the frame period of an imageframe to an expected period.

The present disclosure performs the motion detection according to theimage frames with a frame period thereof being adjusted so as toeliminate the interference caused by the rolling flicker. Furthermore,as the clock frequency is not adjusted, the exposure interval is notchanged to stabilize average brightness of the image frames.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects, advantages, and novel features of the present disclosurewill become more apparent from the following detailed description whentaken in conjunction with the accompanying drawings.

FIG. 1 is a schematic block diagram of an image sensor according to oneembodiment of the present disclosure.

FIG. 2A is a schematic diagram of bright-dark distribution patterns oftwo image frames acquired by an image sensor according to one embodimentof the present disclosure.

FIG. 2B is a schematic diagram of a normalized difference distributionbetween two image frames acquired by an image sensor according to oneembodiment of the present disclosure.

FIG. 3 is an operational schematic diagram of an image sensor accordingto one embodiment of the present disclosure.

FIG. 4 is an operational schematic diagram of an image sensor accordingto another embodiment of the present disclosure in which a total numberof lines per frame is increased by 1.

FIG. 5 is an operational schematic diagram of an image sensor accordingto an alternative embodiment of the present disclosure in which a totalnumber of lines per frame is decreased by 1.

DETAILED DESCRIPTION OF THE EMBODIMENT

It should be noted that, wherever possible, the same reference numberswill be used throughout the drawings to refer to the same or like parts.

One objective of the present disclosure is to provide an image sensorthat eliminates rolling flickers or moving flickers in successive imageframes under a time-varying ambient light. The rolling flickers can beseen through a display screen on which the successive image frames areshown. For example, when using a camera of a smartphone to photograph aspace with the time-varying ambient light, the rolling flickers can beseen on the screen of smartphone.

The image sensor of the present disclosure further utilizes anoscillation frequency of the ambient light to fine tune a frame ratethereof to compensate the clock frequency shift caused by environmentalchange.

Referring to FIG. 1, it is a schematic block diagram of an image sensor100 according to one embodiment of the present disclosure, whichincludes a pixel array 11 and a processor 13. The image sensor 100 is aCMOS image sensor or a CCD image sensor without particular limitations.The processor is a digital signal processor (DSP) or an applicationspecific integrated circuit (ASIC), which processes image framesoutputted by the pixel array 11 using software and/or hardware.

The image sensor 100 of the present disclosure acquires the image framesusing a frame period. e.g., FIG. 1 showing a first image frame IF1 and asecond image frame IF2 captured at different times. According todifferent applications, the first image frame IF1 and the second imageframe IF2 are two adjacent image frames or two image frames separated byat least one another image frame.

Referring to FIG. 3, it is an operational schematic diagram of an imagesensor 100 according to one embodiment of the present disclosure. Thepixel array 11 is exposed using a rolling shutter. One frame period isformed by line times of multiple dark pixel rows Lt1, lines times ofmultiple valid pixel rows Lt2 and line times of multiple dummy pixelrows Lt3, wherein the line times Lt1. Lt2 and Lt3 are identical to eachother, and each of the line times Lt1, Lt2 and Lt3 is referred to a timedifference between the operation beginning of two adjacent pixel rows toform the rolling shutter. In the present disclosure, the line times Lt1,Lt2 and Lt3 are illustrated by a number of clock periods of a clocksignal.

The line times Lt1 cover a time interval for the pre-processing beforepixel data of an image frame starts to be acquired. The line times Lt2cover a time interval for actually acquiring pixel data. The pixel array1 includes physical pixel rows respectively corresponding the operationof the line times Lt1 and the line times Lt2. The difference between thetwo pixel rows is that pixels corresponding to the line times Lt1 arenot illuminated by external light (e.g., covered by an opaque layer) andthus only dark pixel data is outputted.

The line times Lt3 are arranged to adjust the frame period to match apredetermined frequency, e.g., 60 Hz, 50 Hz or multiples thereof. Thepixel array 11 does not include physical pixel rows corresponding to theoperation of the line times Lt3. That is, within the line times Lt3, thepixel array 11 does not generate any pixel data. The line times Lt3 arecontrolled by a dummy row address by, for example, a row decoder forwaiting an end of a current image frame and entering a next image frame.Accordingly, the line times L3 form a blank time interval between twosuccessive image frames.

As shown in FIG. 3, the operation of every pixel row includes anunexposed interval Δt1 (filled with inclined lines), an exposed intervalΔt2 (filled with dots) and a read interval Δt3 (filled with meshes),wherein if the frame period is not adjusted, Δt1+Δt2+Δt3 is fixedcorresponding to successive image frames. The exposed interval Δt2 isdetermined according to the auto exposure, and a length of the exposedinterval Δt2 is changed in conjunction with shortening or extending theunexposed interval ΔtL1 to keep Δt1+Δt2+Δt3 unchanged. In the case thataverage ambient light does not change, the exposed interval Δt2 is notchanged. Accordingly, if it is desired to change the whole frame periodby increasing or decreasing a number of line times of dummy pixel rowsLt3, this objective is implemented by increasing or decreasing theunexposed interval Δt1, illustrated by an example below.

The image sensor 100 further includes a clock generator 12 used togenerate a clock signal CLK, used as a local clock, to count each of theline times of dark pixel rows Lt1, line times of valid pixel rows Lt2and line times of dummy pixel rows Lt3. For example, it is assumed thatthe clock signal CLK is set as 12 MHz as well as Lt1, Lt2 and Lt3 areset as 776 clock periods. In order to generate a VGA image, a number ofline times of valid pixel rows Lt2 is arranged as 488 (includingadditional 8 pixel rows for the image signal processing), and it is alsoassumed that a number of line times of dark pixel rows Lt1 is 10. Inthis case, when a variation frequency of ambient light is 60 Hz and if anumber of line times of dummy pixel rows Lt3 is arranged as 17, the lineper frame (Lpf) is equal to 515 (=10+488+17) such that the frame periodis equal to 1/30 second to match the ambient light variation, i.e.(776/12M)×515= 1/30.

When the frame period does not match the ambient light variation,rolling flickers appear in successive image frames acquired by the imagesensor 100. To not change average brightness of the image frames, thepresent disclosure adjusts a blank time interval between two successiveimage frames by the processor 13 without adjusting a clock frequency ofthe clock signal CLK. Accordingly, even though the clock signal CLKgenerated by the clock generator 12 changes with the temperaturevariation, the frame rate is adjusted to match a predetermined frequencywithout changing the clock frequency.

In one aspect, the blank time interval is determined by a number of linetimes of dummy pixel rows Lt3 by counting a number of pulses of ahorizontal synchronization signal (Hsync shown in FIGS. 3-5) using a rowcounter. In another aspect, the blank time interval is determined by anumber of times of counting the blank time interval by the clock signalCLK generated by the clock generator 12. In this case, since a number ofL3 is not used to determine the blank time interval, it is possible toadjust the blank time interval using a scale smaller than Lt3 (smallestadjustment is a period of the clock signal). In an alternative aspect,the blank time interval is determined by a period of every pulse of thehorizontal synchronization signal Hsync. If the period of every pulse ofHsync is longer, the blank time interval is longer.

It is noted that when rolling flickers exist, bright-dark distributionpatterns between different image frames are different from each other.Accordingly, in one non-limiting aspect, the processor 13 is used tocalculate a first bright-dark distribution pattern of the first imageframe IF1 and calculate a second bright-dark distribution pattern of thesecond image frame IF2. For example, the processor 13 takes thebright-dark variation of at least one pixel column of the first imageframe IF1 as the first bright-dark distribution pattern, and takes thebright-dark variation of the corresponding column of the second imageframe IF2 as the second bright-dark distribution pattern. If the rollingflickers exist, the first bright-dark distribution pattern is differentfrom the second bright-dark distribution pattern since they arebright-dark variations at different times.

In another non-limiting aspect, the processor 13 calculates a summationor an average of gray values of every pixel row of the first image frameIF1 as the first bright-dark distribution pattern (e.g., L1 shown inFIG. 2A), and calculates a summation or an average of gray values ofevery pixel row of the second image frame IF2 as the second bright-darkdistribution pattern (e.g., L2 shown in FIG. 2A). L1 and L2 in FIG. 2Aare shown to be different for illustration purposes.

The processor 13 compares the first bright-dark distribution pattern L1with the second bright-dark distribution pattern L2 to confirm whetherrolling flickers exist or not. For example, the processor 13 calculatesthe similarity or correlation between the first bright-dark distributionpattern L1 and the second bright-dark distribution pattern L2 to performthe comparing process. Referring to FIG. 2A again, if there is norolling flicker, L and L2 are substantially identical to have a highsimilarity and correlation. When the similarity or correlation is lower,it means that the rolling flicker is more serious.

In another aspect, the processor 13 does not compare the whole firstbright-dark distribution pattern L1 with the whole second bright-darkdistribution pattern L2. For example, the processor 13 respectivelyselects at least one (not all) brightest or darkest row(s), but notlimited to, in the first bright-dark distribution pattern L1 and thesecond bright-dark distribution pattern L2. Then, the processor 13compares a position of the selected brightest or darkest row(s) in thefirst and second bright-dark distribution patterns L1 and L2 todetermine whether a position difference between the selected brightestor darkest row(s) is larger than a difference threshold. When theposition difference is larger, it means that the rolling flicker is moreserious. When the position difference is smaller than a differencethreshold, L1 and L2 are considered to be identical.

In the embodiment shown in FIG. 1, the processor 13 includes rowaveraging circuits (shown as row ave.) 131 and 133, a pattern calculatecircuit 135, an analyze circuit (shown as analyze ckt) 137 and an adjustcircuit (shown as adjust ckt) 139. The row averaging circuits 131 and133 are used to respectively calculate the summation or average of grayvalues of every pixel row of the first image frame IF and the secondimage frame IF2. The pattern calculate circuit 135 is used to calculatea first bright-dark distribution pattern (e.g., L1) of the first imageframe IF1 and a second bright-dark distribution pattern (e.g., L2) ofthe second image frame IF2. The analyze circuit 137 is used to comparethe first bright-dark distribution pattern L1 and the second bright-darkdistribution pattern L2 to confirm whether there are rolling flickers.The adjust circuit 139 is used to adjust the frame rate to match apredetermined frequency when the existence of rolling flickers isidentified.

It should be mentioned that although FIG. 1 shows two row averagingcircuits 131 and 133, it is only intended to illustrate but not to limitthe present disclosure. In other aspect, the processor 13 includes onlyone row averaging circuit to calculate the summation or average of grayvalues of every pixel row of both the first image frame IF1 and thesecond image frame IF2.

It is appreciated that although FIG. 1 shows different functions of theprocessor 13 by different blocks, it is only intended to illustrate bynot limit the present disclosure. The operation of every functionalblock is considered to be executed by the processor 13.

In another non-limiting aspect, the processor 13 is used to calculate anormalized difference distribution (P1-P2) between the first bright-darkdistribution pattern L1 and the second bright-dark distribution patternL2 to perform the comparing process. The normalized differencedistribution (P1−P2) is calculated, for example, by a formula2(L1−L2)/(L1+L2) shown in FIG. 2B, wherein (L1+L2) is for normalizingthe difference 2(L−L2). In the present disclosure, when the normalizeddifference distribution (P1−P2) fluctuates between positive and negativevalues, and has a fluctuation amplitude larger than an amplitudethreshold, the processor 11 identifies the existence of rollingflickers. In addition, if there is interference within the field of view(FOV) of the image sensor 100, the processor 13 identifies the existenceof rolling flickers further when a fluctuation time of the normalizeddifference distribution (P1−P2) is smaller than a predetermined numberof times (e.g., FIG. 2B shown about 3.725 times, which is determinedaccording to different system parameters and ambient light frequencies).

In other aspects, the processor 13 further compares two normalizeddifference distributions of two pairs of image frames. When the twonormalized difference distributions have a difference therebetween, itmeans that the rolling flickers exist.

More specifically, in the present disclosure, the objective of adjustinga frame rate is achieved by adjusting, as one way, a number of linetimes of dummy pixel rows Lt3 according to a difference between a firstbright-dark distribution pattern and a second bright-dark distributionpattern of two image frames captured at different times. In the casethat the similarity or the correlation is used to confirm the rollingflickers, the processor 13 further calculates similarities andcorrelations between the first and second bright-dark distributionpatterns of two image frames when said two image frames are separated byimage frames of different separated numbers n (e.g., FIG. 1 showing nbeing selected by a sel. logic) to obtain a separated number ncorresponding to a smallest similarity or a smallest correlation, i.e.for enlarging the difference. The purpose of selecting a best separatednumber n is to cause the feature of FIGS. 2A and 2B to be more apparentfor making the identification easier. In the present disclosure, theseparated number n is selected as 1, 2 or 3, but not limited to.

As mentioned above, when the clock frequency of the clock generator 12changes with the temperature variation, even though Lpf=515 and everyline time (including Lt1, Lt2 and Lt3) is set as 776 clock periods under12 MHz clock frequency, the operating frame period is not equal to 1/30second since the clock frequency is not accurate. Accordingly, thepresent disclosure utilizes the processor 13 to adjust a blank timeinterval to compensate the incorrectness of frame period due to theclock frequency drift. In the present disclosure, the processor adjuststhe blank time interval by adjusting at least one of a number of linetimes of dummy pixel rows Lt3 (i.e., changing a number of dummy pixelrows), adjusting a number of times of counting the blank time by theclock signal and adjusting a period of a horizontal synchronizationsignal Hsync within the blank time.

In the case of adjusting the blank time interval by adjusting a periodof a horizontal synchronization signal Hsync, the period of pulses notwithin the blank time is also adjusted or not adjusted. Morespecifically, different means is used to change the frame rate to matcha variation frequency of ambient light without limited to only changingthe blank time interval.

In this way, the image sensor 100 of the present disclosure can maintaina correct frame rate by adjusting the blank time interval (e.g., a totalnumber of Lpt) even though the clock period is changed by temperaturevariation.

As mentioned above, the processor 13 identifies whether the clockfrequency is changed according to bright-dark distribution patterns ofdifferent image frames, e.g., calculating the similarity or correlationbetween the bright-dark distribution patterns L1 and L2 of differentimage frames as shown in FIG. 2A, or calculating a normalized differentdistribution (P1−P2) of the bright-dark distribution patterns L1 and L2between different image frames as shown in FIG. 2B. Meanwhile, toincrease the identification accuracy, the processor 13 further changes anumber of image frames between said different image frames duringconfirming whether the clock frequency is changed to select the value ofan appropriate separated number n (referring to FIG. 1).

Referring to FIG. 4, when the clock frequency is shifted to increase (orclock period decreased) due to the environmental variation, a total timeinterval for oscillating identical number of times (e.g. 776) isshortened. Accordingly, the present disclosure increases a number ofline times of dummy pixel rows Lt3, e.g., FIG. 4 showing one line timebeing increased to cause Lpf=516, to compensate the deviation of frameperiod caused by the clock frequency drift.

Referring to FIG. 5, when the clock frequency is shifted to decrease (orclock period increased) due to the environmental variation, a total timeinterval for oscillating identical number of times (e.g. 776) isextended. Accordingly, the present disclosure decreases a number of linetimes of dummy pixel rows Lt3, e.g., FIG. 5 showing one line time beingdecreased to cause Lpf=514, to compensate the deviation of frame periodcaused by the clock frequency drift.

The adjusting process of the present disclosure is arranged ascontinuously increasing a number of line times of dummy pixel rows Lt3till a predetermined upper limit (e.g., Lpf=530) is reached when adifference between the first and second bright-dark distributionpatterns exceed a predetermined value. If said difference is notdecreased to be within the predetermined value by increasing the numberof line times of dummy pixel rows Lt3, the number of line times of dummypixel rows Lt3 is then decreased till a predetermined lower limit (e.g.,Lpf=500) is reached to accordingly find a better number of 3. Theadjusting sequence is exchangeable. In addition, when a direction of thefrequency drifting caused by the temperature variation is known, it ispossible to select whether to increase or decrease Lpf at first toreduce the adjusting interval. The line time adjustment of the presentis performed corresponding to a single or fixed ambient light variationfrequency.

It is appreciated that values in the above descriptions such as theclock frequency, line time and a number thereof, and pixel array sizeare only intended to illustrate but not to limit the present disclosure.

As mentioned above, it is known that in a time-varying environmentrolling flickers can appear in successive image frames when a frame rateof the image sensor does not match a variation frequency of ambientlight to degrade the detection accuracy. One reason to cause themismatch is the drifting of a clock period of the image sensor to bedifferent from the expected clock period. Accordingly, the presentdisclosure further provides an image sensor (e.g. FIG. 1) that changes aframe rate of the image sensor without correcting a clock frequencythereof. The reason of not correcting the clock frequency is thataverage brightness of image frames is also changed when the clockfrequency is adjusted. When the frame rate is adjusted to match avariation frequency of ambient light, even though the image framescaptured by the image sensor may still contain fixed flickers, thosefixed flickers do not affect the flowing judgement, e.g., gesture ormotion identification.

Although the disclosure has been explained in relation to its preferredembodiment, it is not used to limit the disclosure. It is to beunderstood that many other possible modifications and variations can bemade by those skilled in the art without departing from the spirit andscope of the disclosure as hereinafter claimed.

What is claimed is:
 1. An image sensor, configured to acquire imageframes with a frame period, the image sensor comprising: a pixel arrayconfigured to output a first image frame and a second image frame,wherein the pixel array is exposed using a rolling shutter; and aprocessor configured to calculate a first bright-dark distributionpattern of the first image frame, calculate a second bright-darkdistribution pattern of the second image frame, calculate a normalizeddifference distribution between the first bright-dark distributionpattern and the second bright-dark distribution pattern, and confirmexistence of rolling flickers when the normalized differencedistribution fluctuates between positive and negative values and has afluctuation amplitude larger than an amplitude threshold.
 2. The imagesensor as claimed in claim 1, wherein the first image frame and thesecond image frame are two adjacent image frames or two image framesseparated by at least one another image frame.
 3. The image sensor asclaimed in claim 1, wherein the first bright-dark distribution patternis a distribution of a summation or an average of gray values of everypixel row of the first image frame, and the second bright-darkdistribution pattern is a distribution of a summation or an average ofgray values of every pixel row of the second image frame.
 4. The imagesensor as claimed in claim 1, wherein the processor is furtherconfigured to confirm the existence of the rolling flickers when thenormalized difference distribution and has a fluctuation time smallerthan a predetermined number of times.
 5. The image sensor as claimed inclaim 1, wherein the processor is configured to calculate a similarityor a correlation between the first bright-dark distribution pattern andthe second bright-dark distribution pattern to calculate the normalizeddifference distribution.
 6. The image sensor as claimed in claim 1,wherein the frame period comprises line times of multiple valid pixelrows, line times of multiple dummy pixel rows and line times of multipledark pixel rows, and the processor is further configured to adjust anumber of the line times of multiple dummy pixel rows when a differencebetween the first bright-dark distribution pattern and the secondbright-dark distribution pattern is larger than a difference threshold.7. An image sensor, configured to acquire image frames with a frameperiod, the image sensor comprising: a pixel array configured to outputa first image frame and a second image frame, wherein the pixel array isexposed using a rolling shutter, and the frame period comprises linetimes of multiple valid pixel rows and line times of multiple dummypixel rows; and a processor configured to calculate a first bright-darkdistribution pattern of the first image frame, calculate a secondbright-dark distribution pattern of the second image frame, and adjust ablank time according to a difference between the first bright-darkdistribution pattern and the second bright-dark distribution pattern. 8.The image sensor as claimed in claim 7, wherein the first image frameand the second image frame are two adjacent image frames or two imageframes separated by at least one another image frame.
 9. The imagesensor as claimed in claim 7, wherein the first bright-dark distributionpattern is a distribution of a summation or an average of gray values ofevery pixel row of the first image frame, and the second bright-darkdistribution pattern is a distribution of a summation or an average ofgray values of every pixel row of the second image frame.
 10. The imagesensor as claimed in claim 7, wherein the processor is configured tocalculate a similarity or a correlation between the first bright-darkdistribution pattern and the second bright-dark distribution pattern toadjust the blank time.
 11. The image sensor as claimed in claim 10,wherein the processor is further configured to calculate similaritiesand correlations between the first bright-dark distribution pattern andthe second bright-dark distribution pattern when the first and secondimage frames are separated by different numbers of image frames toobtain a separated number corresponding to a smallest similarity or asmallest correlation.
 12. The image sensor as claimed in claim 7,further comprising a clock generator configured to generate a clocksignal to count each of the line times of multiple valid pixel rows andthe line times of multiple dummy pixel rows, wherein the processor isconfigured to not adjust a clock frequency of the clock signal whileadjusting the blank time.
 13. The image sensor as claimed in claim 12,wherein the processor is configured to adjust the blank time byadjusting at least one of a number of the line times of multiple dummypixel rows, a number of times of counting the blank time by the clocksignal, and a period of a horizontal synchronization signal within theblank time.
 14. An image sensor, configured to acquire image frames witha frame period, the image sensor comprising: a pixel array configured tooutput successive image frames using a rolling shutter, wherein theframe period of the image frames comprises line times of multiple validpixel rows and line times of multiple dummy pixel rows; a clockgenerator configured to use a clock frequency to count each of the linetimes of multiple valid pixel rows and the line times of multiple dummypixel rows; and a processor configured to adjust a blank time tocompensate a deviation of the frame period caused by a clock frequencyshift when the clock frequency of the clock generator is changed by atemperature variation.
 15. The image sensor as claimed in claim 14,wherein the processor is configured to identify whether the clockfrequency is changed according to bright-dark distribution patterns ofdifferent image frames.
 16. The image sensor as claimed in claim 15,wherein the processor is configured to calculate a similarity or acorrelation of the bright-dark distribution patterns of the differentimage frames to identify whether the clock frequency is changed.
 17. Theimage sensor as claimed in claim 15, wherein the processor is configuredto calculate a normalized difference distribution between thebright-dark distribution patterns of the different image frames toidentify whether the clock frequency is changed.
 18. The image sensor asclaimed in claim 15, wherein the processor is further configured tochange a separated number of image frames between the different imageframes while identifying whether the clock frequency is changed.
 19. Theimage sensor as claimed in claim 14, wherein when the clock frequency isincreased, the processor is configured to increase the blank time; andwhen the clock frequency is decreased, the processor is configured todecrease the blank time.
 20. An image sensor, configured to acquireimage frames with a frame period, the image sensor comprising: a pixelarray configured to output a first image frame and a second image frame,wherein the pixel array is exposed using a rolling shutter; and aprocessor configured to calculate a first bright-dark distributionpattern of the first image frame, calculate a second bright-darkdistribution pattern of the second image frame, and compare the firstbright-dark distribution pattern and the second bright-dark distributionpattern to confirm existence of rolling flickers, wherein the frameperiod comprises line times of multiple valid pixel rows, line times ofmultiple dummy pixel rows and line times of multiple dark pixel rows,and the processor is further configured to adjust a number of the linetimes of multiple dummy pixel rows when a difference between the firstbright-dark distribution pattern and the second bright-dark distributionpattern is larger than a difference threshold.